Grape pickers walk between the vines as they take a break at the Denbies Wine Estate in Dorking, southern England, October 22, 2009.[Agencies]

DORKING, England - The pickers working their way along the hillside, clipping bunches of small, dark purple grapes from the rows of vines and dropping them into plastic buckets are harbingers of a warmer planet.

In recent years, aided by milder springs and autumns, a few British wineries have revived a red winemaking tradition which died around 600 years ago.

Wine aficionados are mixed about the results so far, but say the finest red wines may in future come from north of the English channel if a 190-nation conference in Copenhagen next month fails to agree a strong new U.N. climate change pact.

"We've benefited from global warming," said Chris White, General Manager of Denbies Vineyard, 24 miles south of London, watching plastic trays of Pinot Noir grapes being emptied into a stainless steel wine press in his winery.

"Climate determines the grape varieties you can grow."

Climate scientists have warned that global warming will shift growing patterns for crops, to the point that some developing countries may become too hot or dry to grow enough wheat and maize to feed themselves.

Most experts are too cautious to claim evidence of this theory in actual crop failures caused by droughts or flooding.

However, winemakers are clear that rising temperatures have already redrawn the international wine map, with wine regions developing characteristics of areas further to the south.

"Burgundy has got bigger and riper," wine writer Robert Joseph told Reuters. "Alsace, in North East France, which used to make very light red wine, now makes much fuller red wine. Germany which used to very light red wine, is now making fuller red wine."

BELLWETHER CROP

Wine is especially sensitive to weather and temperature because its only value lies in its taste.

If it's too cold, grapes will not develop fruity flavors or produce enough sugar, giving a wine that tastes acidic.

However, if it is too warm, the grapes produce too much sugar, giving a wine that tastes jammy and heavy.

"For a wine to work a grape should have a harmonious balance of sugar and acidity," said Simon Field, English wine buyer at wine merchant Berry Brothers and Rudd.

Sparkling wines are the most forgiving of under-ripe grapes: red wines need warmth and sunshine the most.